Hard disk drives are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model was established approximately 50 years ago and resembles a phonograph. That is, the hard drive model includes a storage disk or hard disk that spins at a standard rotational speed. An actuator arm with a suspended slider is utilized to reach out over the disk. The arm carries a head assembly that has a magnetic read/write transducer or head for reading/writing information to or from a location on the disk. The complete head assembly, e.g., the suspension and head, is called a head gimbal assembly (HGA).
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are circumferential tracks evenly spaced at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Over the years, the disk and the head have undergone great reductions in their size. Much of the refinement has been driven by consumer demand for smaller and more portable hard drives such as those used in personal digital assistants (PDAs), MP3 players, and the like. For example, the original hard disk drive had a disk diameter of 24 inches. Modern hard disk drives are much smaller and include disk diameters 3.5 to 1 inches (and even smaller 0.8 inch). Advances in magnetic recording are also primary reasons for the reduction in size.
However, the decreased track spacing and the overall reduction in HDD component size and weight in collusion with the load/unload drive capabilities have resulted in problems with respect to the HGA in general and the slider suspension in particular. Specifically, as the component sizes shrink, a need for tighter aerial density arises. In other words, the HGA is brought physically closer to the magnetic media. In some cases, the HGA will reach “ground zero” or contact recording. However, one of the major problems with near contact recording is the effect of vibration resonance when a slider encounters the asperities of the magnetic media or disk.
For example, when the slider contacts the disk, dynamic coupling between the slider and components of the head gimbal assembly (including the gimbal structure and nose portion) make the interface unstable and generate a strong or even a sustained slider (or even HGA) vibration. The vibration will result in slider flying high for some period of time and than returning to normal fly-height as the vibrations reduce below a threshold. This phenomenon is known as fly-height hysteresis. When slider is flying higher or lower than normal, the read/write performance is degraded. In other words, if the fly-height hysteresis occurs during a data write operation, the data is not written to the disk properly due to the higher than normal head to disk distance, and in the worst case, the data is lost. Similar difficulties arise during a read operation.
Thus there is a need for monitoring the fly-height of a read-write head and for detecting collisions with asperities that could introduce momentary fly-height modulations, and for compensating for them by informing the read-write controller when they have occurred.